This invention relates to a method for producing a thin film and, more particularly, to a method for forming a thin film on a wafer by chemical reaction under heat with a single wafer process.
There has hitherto been known a method of producing a thin film on a wafer, by chemical reactions under heat, using a low-pressure chemical vapor deposition(termed as LP-CVD) system.
In forming a thin film on a wafer, with a single wafer process by using a LP-CVD system, the following procedure is used. First, before proceeding to processing, the inside of a process chamber is evacuated to a low pressure and heated until the atmosphere in the chamber is stabilized at a preset temperature, in order to make preparations for film deposition.
As the evacuated state is maintained, a single wafer is loaded into the chamber and held on a susceptor. Mixed gases are then introduced into the inside of the chamber. By introducing gases under a heated environment, a film deposition on the wafer is started and a thin film is formed on the wafer surface.
As the low pressure state in the chamber is maintained, the wafer, on which the thin film of a preset thickness has now been formed, is unloaded to outside the chamber and cooled. After taking out the wafer, the next wafer is loaded into the chamber and placed on the susceptor. The deposition of a thin film on the wafer and taking the wafer out of the chamber are sequentially executed. This sequence of operations then is carried out for each wafer.
It should be noted that, if, in the above-described a thin film forming method using a single wafer process, an LP-CVD device, on which the film-deposition processing has not been carried out for a prolonged time, heat dissipation from within the chamber is suppressed, while the power necessary for maintaining the preset temperature within the chamber is turned off, so that the atmosphere in the chamber is stabilized only gradually as from the time the film-forming process is started and hence a lot of time is taken until the atmosphere in the chamber is stabilized.
This situation also persists in case the LP-CVD unit is used next to cleaning and pre-coating or next to changing the working process temperature.
If the atmosphere in the chamber is stabilized only after lapse of long time, the film formed on the wafer surface becomes thin or it becomes impossible to attain the uniformity in the film thickness, at an initial state, when the film forming process is started, and hence film forming tends to be unstable. This problem is thought to be of particular significance. in consideration that, when the LP-CVD system, the need for which is increasing recently, is used for a temperature sensitive process, temperature control performs a crucial role in film thickness management.
In order to cope with this situation, the conventional practice has been to perform an advance film-forming processing (termed preparatory processing), using dummy wafers, for a time interval necessary for stabilizing the atmosphere within the chamber, prior to prosecution of the processing for forming a film on a wafer which is to be a product. The principal film forming processing (termed product processing) is then carried out after the atmosphere in the chamber has become stable.
FIG. 8 shows a timing chart for illustrating the conventional film forming process employing dummy wafers. In FIG. 8, preparatory processing for processing dummy wafers and product processing of processing a wafer which is to be a product, are first carried out in this order.
The preparatory processing is carried out by the following steps. On starting the process, a number of dummy wafers are loaded over about three minutes and the inside of a load-lock chamber is evacuated for about three minutes. The first dummy wafer is processed for about five minutes. The processing for the second wafer, the third wafer and so on is then carried out so that a sum total of 25 dummy wafers are processed.
The 25 wafers represent an ad-hoc measure in dummy wafer processing for accomplishing a stable film thickness in case of a single wafer process in which a single wafer is taken in and out one by one. Thus, the dummy wafer processing comes to a close in about 125 minutes, assuming that processing of each dummy wafer takes about five minutes, or in 131 minutes as from the start of processing.
After the end of the dummy wafer processing, the inside of the load-lock chamber is opened to atmosphere for about three minutes, after which the dummy wafers are unloaded in about three minutes. Thus, the preparatory processing of processing the dummy wafers takes about 137 minutes.
The product processing is carried out by the following process steps. Simultaneously with the start of the evacuation of the load-lock chamber in the preparatory processing, the processing is started and, after loading the wafer in approximately three minutes, the load-lock chamber is evacuated for about three minutes.
After the end of the dummy wafer processing in the preparatory processing, that is, after about 131 minutes as from the start of the processing, wafer processing is started. After processing the first wafer for about five minutes, the second wafer, the third wafer and so on are processed.
That is, since the film-forming process is started after the atmosphere in the chamber is stabilized by the processing of 25 dummy wafers, there is no risk that the thickness of the film is reduced or that the uniformity in the film thickness cannot be maintained.
However, since 25 dummy wafers need to be processed as the preparatory processing before processing the products, the entire processing time necessary for forming a film of a wafer as a product is prolonged. This leads to the lowered operating ratio of the LP-CVD system to raise the wafer production cost.
Accordingly, it is an object of the present invention to provide a method for producing a thin film whereby the process of forming a thin film on a wafer to be a product is carried out efficiently to shorten the process time and to raise the operating ratio of a film forming device.
The above and other objects of the present invention are satisfied, at least in part, by providing a method for forming a thin film on a wafer by chemical reaction under heat with a single wafer type process, said method comprising: a pseudo-process provided after starting a process of forming a thin film, said pseudo-process operating to suppress variations in the film thickness caused by temperature in a reaction chamber.
In accordance with the present invention, a thin film is formed on the wafer by chemical reaction under heat, following the start of the thin film forming process, through a pseudo process of suppressing variations in the film thickness caused by temperatures within the reaction temperature. This enables the processing of forming a thin film on a wafer which is to be a product to execute efficiently for reducing the process time and hence the operating ratio of the film forming device is enhanced.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description in conjunction with the accompanying drawings wherein only the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out this invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.